Summer Days is an alternate universe prequel to the game School Days, set in the summer holidays just prior to the timeline of School Days. All the original characters are present, except it assumes that they all first meet during the summer holidays as opposed to during the first year of school. Although Kotonoha and Sekai, the heroines of the first game, are both present, the main heroine is Setsuna.

I got this From Overflow's Official Website...(The creation team that created school days)

The main heroine in the story is Setsuna, whom has a secret crush on Makoto. Her good friend Sekai has a summer job as a waitress in a family restaurant called "Radish". However, Sekai has become sick and needs to stay in bed. Setsuna stands in for Sekai, and by chance she met Makoto near the seaside and thus their story starts.